The first results on subharmonics were
obtained by Rabinowitz in [1]}, who showed the existence of
infinitely many subharmonics both in the subquadratic and superquadratic
case, with suitable growth conditions on \(H^{\prime }\) . Again the duality
approach enabled Clarke and Ekeland in [1]} to treat the
same problem in the convex-subquadratic case, with growth conditions on
\(H\)  only.
